1. Field of the Invention
The present invention generally relates to the art of wireless communications system. In particular, the present invention relates to minimizing power consumption in mobile stations.
2. Description of the Related Art
A. Basic Wireless System Architecture.
As shown in FIG. 1, a cellular network is comprised of three fundamental parts:
(1) a mobile station 1 (which is carried by the subscriber); PA1 (2) a base station subsystem 2 (which controls the radio link with the mobile station); and PA1 (3) a network subsystem 3 (which is interfaced to the public fixed network 4 and the base station subsystem).
The network subsystem and the base station subsystem communicate across an interface 5, while the mobile station and the base station subsystem communicate using a radio link 6.
Mobile Station. The mobile station is the "phone" part of the wireless communication system. The mobile station may be fixed or portable. Fixed mobile stations are permanently installed in a car or a stationary location. Portable units include bag phones and hand-portable phones (commonly called "cell phones"). Hand-portable phones are becoming increasingly popular because they can be carried easily on the person of the subscriber. A mobile station includes an antenna 7 for transmitting and receiving radio signals from the base station subsystem.
Base Station Subsystem. The base station subsystem comprises two fundamental elements, (1) one or more base transceiver stations (8 and 9) and (2) a base station controller 10. These components communicate across another interface 11. A base transceiver station includes radio transceivers that handle radio-link protocols with the mobile station and an antenna 12 for communication with mobile stations.
The base station controller manages the radio resources of the base transceiver stations. It also manages handovers (passing the audio from cell to cell during a call), frequency hopping (changing operating frequency to maintain signal quality) and radio-channel setup.
Network Subsystem. The basic element of the network subsystem is the mobile services switching center (MSC) 13. The MSC is the interface of the cellular network to the public fixed network and, as such, basically performs the functions of a switching node of the public fixed network. The MSC also routes calls from the public fixed network (via the base station controller and the base transceiver station) to the mobile station. The MSC also provides the wireless system with individual information about the various mobile stations and performs the functions of authentication, location updating, and registration. The MSC may operate in conjunction with other functional entities which further comprise a network subsystem, such as registers which hold information regarding current mobile station location and subscriber information.
B. Radio Link.
In conventional wireless communications technology, user data (e.g. speech) is encoded in a radio frequency for transmission and reception between a base station and a mobile unit. Because the number of available radio frequencies, or "channels," for cellular system is less than the number of all possible users, the system is "trunked." Trunking is the process whereby users share a limited number of channels in some predetermined manner.
A common form of trunked access is the frequency-division multiple access (FDMA) system. In FDMA, the limited channels are shared by all users as needed. However, once a channel is assigned to a user, the channel is used exclusively by the user until the user no longer needs the channel. This limits the number of concurrent users of each channel to one, and the total number of users of the entire system, at any instant, to the number of available channels.
Another common trunking system is the time-division multiple access (TDMA) system. TDMA is commonly used in telephone networks, especially in cellular telephone systems, in combination with an FDMA structure. In TDMA, data (speech) is digitized and compressed to eliminate redundancy and silent periods, thus decreasing the amount of data which is required to be transmitted and received for the same amount of information. Each of the channels used by the TDMA system is divided into "frames" and each of the users sharing the common channel is assigned a time slot within the frames. The TDMA system appears, to each of the users sharing the channel, to have provided an entire channel to each user.
Code-division multiple access (CDMA), yet another common trunking system, is an application of spread spectrum techniques. The main advantage of CDMA systems as compared to TDMA systems is that all the mobile stations can share the full transmission spectrum asynchronously, that is, there is no need for synchronization among mobile stations (only between a mobile station and a base station).
C. Mobile Station Architecture.
As shown in FIG. 2, mobile stations generally comprise two basic parts, the RF (radio frequency) part 20 and the digital part (or baseband processing circuitry) 21. The RF part operates receiving, transmitting, and modulation functions. The digital part handles data processing, control, and signaling functions. As shown, the radio frequency part includes an antenna 27 for receiving and transmitting radio signals. A radio signal received by the radio frequency part is converted to a lower frequency signal and delivered 22 to the digital part. Likewise, a signal generated by the digital part is delivered 23 to the radio frequency part, which in turn converts the signal to a higher frequency signal, and transmits that higher frequency signal.
The digital part is operatively connected to a handset 24, which has a speaker 25 and a mouthpiece 26. All or part of the radio frequency part and the digital part can be disposed within the handset, as is the case with cell phones.
Also included in the mobile station architecture (but not shown in FIG. 2) is a reference clock, which is used to drive the digital hardware. Clock circuitry may also include tuning circuitry or temperature compensation circuitry to make the reference signal more accurate. A control processor performs the control functions of the mobile station, including, for example, power control and the selection of different channels.
For CDMA systems, mobile stations generally include the following elements. Transmitting circuitry transmits as spread spectrum signals data (e.g. speech) provided by a user, while receiving circuitry receives spread spectrum signals and converts the signals into a form intelligible to the user. Pseudorandom noise (PN) sequence generator circuitry operationally connected to the transmitting circuitry and the receiving circuitry enables the mobile station to transmit and receive spread spectrum signals. Prior to transmission, each data bit is spread into a number of "chips" which can be transmitted in a bandwidth-limited channel along with signals of many other users, who can all share the channel. A chip rate clock, operating at a chip rate, clocks the PN sequence generator circuitry.
D. Sleep Circuitry.
One significant challenge facing designers of mobile stations is conserving power. Because mobile stations are generally powered by batteries, mobile stations which consume available power quickly have a significant disadvantage. One manner in which power can be conserved is to introduce sleep circuitry into the mobile station. With sleep circuitry, when the mobile station is in idle mode (i.e., listening to a paging channel periodically, but otherwise taking no action), the control processor commands the mobile station to enter into a sleep mode to minimize power consumption. During sleep mode, portions of the mobile station are shut down, thereby conserving energy. A design goal is to optimally use the sleep mode by turning off as many electronic components as possible.
Moreover, CDMA spread spectrum wireless communication systems require the maintenance of a high timing accuracy during sleep mode, as they employ rapid PN sequences. Since the PN sequences are rapid and the mobile station PN sequence must be time aligned with the transmitted base station sequence, the wake up time of the system must be very accurate. It would be advantageous if the system could power off the high precision, temperature compensated, high speed reference clock used to maintain the PN sequences, using instead a slower, lower power clock with less precision to determine the length of the sleep period.
A problem with current sleep mode circuitry is that, while the mobile station is in sleep mode, the high speed reference clock is still in operation. Because of the high frequency of the reference clock, significant power is consumed even during sleep mode. Additionally, prior techniques for calibrating a low speed clock are not sufficiently accurate for CDMA systems.
Therefore, objects of the present invention include minimizing power consumption in mobile stations by turning off the high speed reference clock and as many other components as possible, and providing for high accuracy calibration of a low speed clock used during sleep mode.